InP-based transistor fabrication

ABSTRACT

Methods of forming structures that include InP-based materials, such as a transistor operating as an inversion-type, enhancement-mode device. A dielectric layer may be deposited by ALD over a semiconductor layer including In and P. A channel layer may be formed above a buffer layer having a lattice constant similar to a lattice constant of InP, the buffer layer being formed over a substrate having a lattice constant different from a lattice constant of InP.

RELATED APPLICATION

This application claims the benefit of and priority to U.S. ProvisionalApplication Ser. No. 60/934,716, filed Jun. 15, 2007, the entiredisclosure of which is incorporated by reference herein.

TECHNICAL FIELD OF THE INVENTION

The technology disclosed herein relates generally to semiconductorstructures and methods of making semiconductor structures and morespecifically to use of alternatives to Si for constructing transistorsusing InP-based materials.

BACKGROUND

The increasing operating speeds and miniaturization of semiconductordevices have given rise to a need to find alternatives to silicon forthe source, drain, and channel regions of transistors and to SiO₂ forthe gate dielectric of FET transistors. Research has been performed oncompound semiconductors such as GaAs and other III-V materials forhigh-performance transistor applications. Although there have beensuccessful demonstrations of depletion-mode GaAsmetal-oxide-semiconductor field-effect transistors (MOSFETs),inversion-type enhancement-mode GaAs MOSFETs have suffered from problemssuch as relatively low drain currents. Notwithstanding research anddevelopment efforts by many institutions into aspects of III-Vmaterials, there remains a need for commercially viable techniques forproviding improved performance using materials such as InP.

SUMMARY

According to aspects of embodiments of the invention described below,atomic layer deposition (ALD) techniques with high-k gate dielectricsare used to provide inversion-type enhancement-mode InP-based MOSFETsthat demonstrate substantial technological improvements. For example,drain currents for InP devices described below may have drain currentsmore than 1,000 times greater than drain currents for existingenhancement-mode GaAs MOSFETs. Other aspects include improvedperformance with respect to Fermi level pinning and higher electronsaturation velocity (2×10⁷ cm/s). At equivalent gate lengths, aspects ofthe embodiments described below can provide high-field transconductancegreatly improved (>50% higher) in comparison to devices havingequivalent channel lengths and fabricated with more conventionalsemiconductors such as Si, Ge, or GaAs. The ALD high-k/InP-basedmaterial system mitigates problems associated with Fermi-level pinningreported in the literature.

Provision of high-performance InP-based MOSFETs may enable a newgeneration of high-frequency transistors because of the intrinsicallyhigher and robust charge-transport properties of InP-based materialsrelative to other semiconducting materials. The presence of a gatedielectric in MOSFET structures reduces overall device leakage andenhances both control and responsivity of channel regions. The need forgate dielectric materials suitable for use with InP, and methods forformation thereof, has hitherto limited the commercialization ofInP-based MOSFETS. Previous efforts focused on the deposition ofdielectric materials on InP have resulted in various difficultiesincluding interfacial chemical instabilities, electronic instabilitiessuch as Fermi-energy pinning, and device fabrication difficultiesinvolving high series-resistance source and drain terminals. Embodimentsof the present invention provide techniques enable the fabrication ofcommercially viable InP-based MOSFETs, a long-sought butyet-to-be-achieved goal.

In an aspect, an embodiment of the invention includes a method offabricating a transistor. An InP-based material layer structureincluding a channel layer is provided. A source region and a drainregion are defined in the InP-based material layer structure. A channelregion is provided between the source and drain regions, the channelregion including at least a portion of the channel layer. A gatedielectric is deposited over the channel region by atomic layerdeposition (ALD). A gate is provided above the channel region. At leasta portion of the gate dielectric may be disposed between the gate andthe channel region, and the transistor may operate as an inversion-type,enhancement-mode device.

One or more of the following features may be included. The source anddrain regions are at least partially disposed in the channel layer. Thesource and drain regions are disposed above the channel layer. The gatedielectric comprises at least one of a group II material and/or atransition metal, e.g., includes at least one of aluminum, Al₂O₃,hafnium, and/or HfO. The channel layer may include a strained region.The channel layer may include at least one of InSb, InGaAs, InAs, and/orInP. The channel layer may include a plurality of substantiallylattice-matched layers.

The transistor may be, e.g., a MOSFET or a HEMT. The InP-based materiallayer may be provided over a silicon substrate.

In another aspect, an embodiment of the invention includes a method offabricating a transistor. An InP-based material layer including achannel layer is provided. A source region and a drain region aredefined in the InP-based material layer. A channel region is providedbetween the source and drain regions, the channel region including atleast a portion of the channel layer. A gate dielectric is formed overthe channel region, the gate dielectric including at least one of agroup II material or a transition metal. A gate is provided above thechannel region. At least a portion of the gate dielectric may bedisposed between the gate and the channel region, and the transistor mayoperate as an inversion-type, enhancement-mode device.

One or more of the following features may be included. The gatedielectric may include aluminum and/or hafnium. The gate dielectric isformed by ALD.

In still another aspect, an embodiment of the invention includes amethod of fabricating a semiconductor structure. A semiconductor layercomprising indium and phosphorous is provided. A dielectric layercomprising hafnium is deposited by ALD above at least a portion of thesemiconductor layer. A conductive layer is provided over at least aportion of the dielectric layer. The dielectric layer is disposedbetween the semiconductor layer and the conductive layer.

One or more of the following features may be included. The semiconductorlayer includes a transistor channel region and the conductive layerincluding a transistor gate region. The dielectric layer may include atleast one of oxygen, silicon, and/or nitrogen. The semiconductor layeris provided over a substrate that includes silicon.

In another aspect, an embodiment of the invention includes asemiconductor structure. The structure includes a silicon substrate. AnInP-based material layer structure including a channel layer is disposedabove the substrate. A transistor including a source region, a drainregion, and an inversion-type channel region may be disposed at leastpartially within the channel layer, and a gate region is disposed abovethe channel region.

One or more of the following features may be included. A mask layer isdisposed above the substrate and includes a plurality of openings; asecond layer of material having a lattice-mismatch with silicon isdisposed within the openings. The second layer may include indium andphosphorous.

A dielectric layer is disposed between the channel region and the gateregion, the dielectric layer including at least one of aluminum and/orhafnium. The channel layer includes a strained region. The channel layerincludes at least one of InSb, InGaAs, InAs, and/or InP. The channellayer includes a plurality of substantially lattice-matched layers.

In still another aspect, an embodiment of the invention includes amethod of making a semiconductor structure. A substrate including a toplayer having a compound comprising indium and a second element isprovided. A dielectric layer is provided over the top layer, a region ofthe dielectric layer adjacent the top layer being substantiallynon-reactive with indium and the second element and configured tosuppress out-diffusion of dopants from the top layer during a thermalanneal step. A dopant species is implanted into the top layer. The toplayer is thermally annealed to activate the dopant at a relatively hightemperature.

One or more of the following features may be included. The secondelement is phosphorous, antimony, aluminum, gallium, and/or arsenic.Implanting a dopant species into the top layer includes implanting thedopant species through the dielectric layer. The dielectric layerincludes first and second layers, the first layer being adjacent the toplayer and substantially non-reactive with indium and the second element,the first layer having a first composition and the second layer having asecond composition different from the first composition. The secondlayer includes oxygen, aluminum, and/or hafnium. The substrate includesa bottom portion including silicon.

In another aspect, an embodiment of the invention includes a method offorming a transistor. The method includes providing a substrateincluding a top layer disposed thereover, the top layer including acompound comprising indium and a second element. A dielectric layer isdeposited above the top substrate layer by atomic layer deposition. Amask layer may be disposed above the dielectric layer to define a gateregion, a source region, and a drain region of a transistor. A dopant isimplanted through the dielectric layer into the source region and thedrain region proximate the gate region, the mask layer blockingimplantation of the dopant into the gate region. A high temperatureanneal is performed to activate the dopant in the source and drainregions.

One or more of the following features may be included. The substrateincludes a bottom portion including silicon. Implanting a dopant throughthe dielectric layer to define the source and drain regionssubstantially amorphizes the source and drain regions. The hightemperature anneal recrystallizes the source and drain regions. Aportion of the mask layer is replaced with a gate structure withoutlithography so that the gate structure is aligned to the source anddrain regions. A substantially lattice-matched layer is provided atopthe substrate in which the source and drain regions are formed. Thelayer may include at least one of InSb, InGaAs, InAs, and/or InP; thelayer may include a plurality of substantially lattice-matched layers.

In another aspect, an embodiment of the invention includes a method offorming a transistor. A substrate including indium and phosphorous isprovided. A mask layer is disposed above the substrate to define a gateregion of a transistor. A dopant is implanted to create source and drainregions, with the mask layer blocking implantation of the dopant intothe gate region. A portion of the mask layer is replaced with a gatestructure without lithography so that the gate structure is aligned tothe source and drain regions.

One or more of the following features may be included. The substrateincludes a bottom portion including silicon. Implanting a dopantsubstantially amorphizes the source and drain regions. A hightemperature anneal is performed to activate the dopant and torecrystallize the source and drain regions.

A high temperature anneal is performed to activate the dopant in thesource and drain regions. The gate structure includes a dielectric layercreated after performing a high temperature anneal to activate thedopant.

In yet another aspect, an embodiment of the invention includes a methodof forming a transistor. A substrate comprising indium and phosphorousis provided. A mask is provided above the substrate to define source,drain, and gate regions of a transistor. A dopant is implanted into thesource and drain regions. At least a portion of the source and drainregions is amorphized. An anneal is performed to recrystallize theamorphized portions of the source and drain regions.

One or more of the following features may be included. The substrateincludes a bottom portion comprising silicon. The substrate includes atop layer comprising indium and phosphorous, InP, InSb, InGaAs and/orInAs. A barrier layer in contact with a top portion of the substrate isprovided. A region of the barrier layer in contact with the top portionof the substrate is substantially free of oxygen and/or includesnitrogen. The barrier layer includes at least one of AlN, TaN, and/orTiN.

The barrier layer includes a dielectric layer relatively inert to thetop portion and the dielectric layer is formed by atomic layerdeposition. A second dielectric layer is provided above the dielectriclayer, with the second dielectric layer being not inert to the topportion of the substrate.

The barrier layer is oxidized to improve a dielectric property of thebarrier layer. The steps of implanting and amorphizing include a singlestep of implanting a dopant that amorphizes at least a portion of thesource and drain regions. The dopant includes SiX, with X including agroup VIIA element. Ohmic contacts adjacent the source and drain regionsare defined.

In still another aspect, an embodiment of the invention includes forminga transistor. A substrate including indium and phosphorous is provided.A mask layer may be formed above the substrate to define a sourceregion, a drain region, and a gate region of a transistor. A dopant isimplanted into the source and drain regions, the mask layer blockingimplantation of the dopant into the gate region. The mask layer isremoved to expose the gate region. At least a portion of a channel layeris epitaxially grown.

One or more of the following features may be included. The portion ofthe channel layer is epitaxially grown by ALD, MBE, or CVD. Epitaxiallygrowing at least a portion of the channel layer includes growing one ormore of InSb, InGaAs, InAs, and/or InP. The gate channel includes indiumand phosphorous; indium and antimony; indium and arsenic; and/or galliumand arsenic.

The substrate includes a top surface prior to the step of forming themask layer, the gate channel layer includes a top surface, and the topsurface of the gate channel layer is disposed above the top surface ofthe substrate. The substrate includes a top surface prior to the step offorming the mask layer, the gate channel layer includes a bottomsurface, and the bottom surface of the gate channel layer is disposedbelow the top surface of the substrate.

In yet another aspect, an embodiment of the invention includes a methodof forming a transistor. The method includes providing a substrateincluding indium and phosphorous. A mask layer may be disposed above thesubstrate to define source, drain, gate, and gate channel regions of atransistor. A first dopant type is provided in the gate channel region.A portion of the substrate in the source and drain regions may beremoved to form source and drain recesses. The source and drain recessesare filled with a semiconductor material having a second dopant typedifferent from the first dopant type.

One or more of the following features may be included. The second dopanttype is opposite from the first dopant type. Filling the source anddrain recesses includes an epitaxial growth technique, e.g., at leastone of MBE, ALD, and CVD. Filling the source and drain recesses includesfilling the source and drain recesses with a material having a latticeconstant different from a lattice constant of the substrate and thematerial induces stress in the channel region. The semiconductormaterial includes at least one of GaAs, InGaAs, In-rich InGaAs, or InP.

In another aspect, an embodiment of the invention includes a method offabricating a transistor. A crystalline substrate having a first latticeconstant substantially different from a lattice constant of InP isprovided. A non-crystalline mask is provided above the substrate, themask including an opening having sidewalls that extend above thesubstrate by a height. A crystalline buffer layer having a secondlattice constant substantially similar to the lattice constant of InPmay be provided within the opening. The buffer layer may have a heightless than the mask opening sidewall height, the buffer layer heightbeing sufficient to permit defects arising from the lattice mismatch toexit the buffer layer at the mask opening sidewalls. A channel layer isprovided above the buffer layer, the channel layer being substantiallylattice-matched with the buffer layer. A source region, a drain region,and a channel region of a transistor are defined in the channel layer. Agate is provided above the channel layer.

One or more of the following features may be included. The substrate isa silicon substrate. The mask includes SiO₂. The buffer layer includes aIII-V material, e.g., InP. The channel layer includes at least one ofInP, InSb, InGaAs, InAs, and/or InP. The channel layer is disposedwithin the mask opening. The channel layer is disposed substantiallycompletely above the mask. The transistor is an inversion-type,enhancement-mode transistor.

In another aspect, an embodiment of the invention includes a method ofprocessing a semiconductor structure. A substrate comprising indium andphosphorous is provided. Oxide from a top surface of the substrate isremoved using at least one of HF, HCl, and NH₄OH. The top surface ispassivated using at least one of an (NH₄)₂S or an OH-group compound.

The removing and passivating steps may be performed in situ or performedex situ.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a)-(b) are schematic cross-sectional views illustrating anembodiment of the invention including the implantation and anneal ofsource and drain regions through a layer that is subsequently removed;

FIGS. 2( a)-(j) are schematic cross-sectional views of enhancement-modeInP MOSFETs fabricate in accordance with embodiments of the invention,and charts describing experimental procedures and data obtained forenhancement-mode InP MOSFETs;

FIGS. 3( a)-(d) are schematic cross-sectional views illustrating thefabrication of InP-based devices on lattice-mismatched substrates suchas silicon, in accordance with an embodiment of the invention;

FIGS. 4( a)-(c) are schematic cross sectional views illustratingfabrication of an enhancement-type InP MOSFET, in accordance with anembodiment of the invention;

FIGS. 5( a)-(g) are schematic cross-sectional views illustrating amethod of making a self-aligned gate structure for an InP-based MOSFET,according to an embodiment of the invention;

FIGS. 6( a)-(b) are schematic cross-sectional views illustratingdielectric layers deposited on an InP substrate; and

FIGS. 7( a)-(d) are schematic cross sectional views illustrating amethod of implantation and amorphization of source and drain regionsthrough a diffusion/barrier layer and subsequent recrystallization ofthe source and drain regions and oxidation to improve the layer'sdielectric properties.

DETAILED DESCRIPTION

Aspects of this invention address, among other things, the challenges offorming a high-quality interface between a gate dielectric and InP andother InP-based materials through the use of ALD. See also Y. Q. Wu, etal., “Inversion-type enhancement-mode InP MOSFETs with ALD Al₂O₃, HfO₂and HfAlO nanolaminates as high-k gate dielectrics,” Proceedings of65^(th) Device Research Conference, Notre Dame, USA (2007) and Y. Q. Wu,et al., “Enhancement-mode InP n-channel metal-oxide-semiconductorfield-effect-transistors with atomic-layer-deposited Al₂O₃ dielectrics,”Applied Physics Letters 91, 022108-022110 (2007), incorporated byreference herein in their entireties.

Referring to FIG. 1( a), a semiconductor substrate 100 has an InP-basedtop layer 110 disposed thereover. In various embodiments, a portion ofthis InP layer may be used to form a channel for a transistor. Thesubstrate 100 may include a semiconductor material and may be, forexample, a bulk silicon wafer, a bulk germanium wafer, asemiconductor-on-insulator (SOI) substrate, or a strainedsemiconductor-on-insulator (SSOI) substrate. The substrate 100 mayinclude or consist essentially of a first semiconductor material, suchas a group IV element, e.g., germanium or silicon. In an embodiment, thesubstrate 100 includes or consists essentially of (100) silicon. In someembodiments, the substrate 100 is an InP substrate, with a top region ofthe InP substrate defining the InP-based top layer 110.

The InP-based top layer 110 may be formed by molecular beam epitaxy(MBE), a chemical vapor deposition (CVD) process, e.g., metalorganic CVD(MOCVD), or other deposition techniques on the substrate 100. In anembodiment in which the InP-based top layer 110 is InP disposed over anInP substrate, the top layer may have a thickness ranging from, e.g., 1nm to 100 μm. In an embodiment, the composition and lattice constant ofthe InP-based top layer 110 is different from the composition andlattice constant of the substrate 100. In a preferred embodiment, thethickness of the InP-based top layer 110 is below its critical thicknessand the InP-based top layer 110, i.e., the channel layer, may include astrained region. For example, with a lattice-mismatch of 5%, thecritical thickness is about 15 nm, and the InP-based top layer 110preferably has a thickness less than 15 nm. In other embodiments, thethickness of the InP-based top layer 110 is greater than the criticalthickness and the layer is at least partially relaxed. In an embodiment,the InP-based top layer 110 is relaxed.

In alternative embodiments, one or more layers of materials that aresubstantially lattice-matched to InP, such as InAlAs or InGaAs, or thinlayers of non-lattice-matched III-V compounds, such as GaAs, InSb orInAs, are provided above the InP-based top layer 110 for use in formingtransistor source, drain, and/or channel regions. Such thin layers ofnon-lattice-matched III-V compounds may be strained, which may result inenhanced performance characteristics such as improved electron and/orhole mobility. As used herein, “substantially lattice-matched to InP”means having a lattice constant difference with a lattice constant ofInP of no greater than about 10%, preferably no greater than about 5%.To maintain strain in non-lattice-matched III-V compounds, such as GaAs,InSb, and InAs, the thickness of such layers is preferably less thanabout 10 nm.

As used herein, the term “InP-based materials” is used to refer tocompositions that include or consist essentially of InP and may includeadditional elements. As used herein, “InP-based material layerstructure” is used to refer to an InP layer or InP-based layer alone orin combination with one or more layers that are substantially latticematched to InP or in combination with thin, strained,non-lattice-matched III-V layers.

To prevent damage to the top layer during ion implantation, such as whencreating transistor source and drain regions, an encapsulationprotection layer 120, i.e., a dielectric layer, is disposed on the topsurface of the InP-based top layer 110. This protection layer allowsimplantation therethrough while preventing damage of the InP-based toplayer. Examples of a suitable protection layer include a SiO₂ or SiNlayer deposited by a CVD process, e.g., MOCVD, or an Al₂O₃ layer or ananolaminate layer, including a material such as HfO₂ and/or HfAlO,deposited by ALD. A mask 130 is formed above the protection layer, andused to define a gate region 140, a source region 150 and a drain region160 for a transistor. The mask forms a dummy gate structure 170, whichserves as a hard mask for a subsequent ion implantation step. The maskmay be formed from a suitable masking material, such as a dielectricmaterial, e.g., Si₃N₄, in accordance with methods known to one of skillin the art.

The source and drain regions 150, 160 are defined in the InP-based toplayer by performing an appropriate source/drain dopant ion implantation180, using conventional techniques known to those of skill in the art,taking into account the composition of the protection layer. A channelregion 190 is provided between the source and drain regions and includesat least a portion of the InP-based top layer 110, i.e., the channellayer. An anneal is then performed at a suitably high temperature toactivate the implanted dopant, for example between about 600 and 900° C.with rapid thermal processing (“RTP”). The encapsulation protectionlayer protects the InP-based material surface during the implantationand anneal processes.

The encapsulation protection layer and the dummy gate structure may beremoved after the implantation and anneal steps to create a device-readyInP-based top layer 110 as shown in FIG. 1( b). Optionally, to improvethe surface quality of the InP-based top layer prior to devicefabrication, a portion of its top surface may be removed to ensure theelimination of any portion that is or may be damaged. Another optionalmethod to improve the quality of the InP-based top layer 110 is toprovide an additional protective/sacrificial cap layer (not shown) onthe top surface InP-based top layer 110 before forming the protectionlayer 120, for example, a passivation layer including one or morematerials such as S, NH₄OH, amorphous Si, and/or amorphous Ge.

Subsequently, the surface of the InP-based top layer 110 is cleaned, anda high-quality gate dielectric layer (not shown) is deposited by, e.g.,an ALD process in, for example, an ASC F-120 ALD reactor. The gatedielectric layer may be a high-k dielectric. Exemplary gate dielectricstructures include, for example, a layer of Al₂O₃, a layer of HfO₂layer, a nanolaminate layer of HfO₂ and Al₂O₃, or HfAlO, or any othersuitable gate dielectric material. A post-deposition anneal (PDA) may beperformed using O₂, N₂ or any other suitable ambient at an appropriatetemperature, for example between about 400 and 700° C.

A gate (not shown) is formed above the channel region. At least aportion of the gate dielectric is disposed between the gate and thechannel region, and the transistor operates as an inversion-typeenhancement-mode device, i.e., a device that is off at a gate voltageV_(g)=0, and is on at V_(g)>0.

As is known to those skilled in the art, there has been a long-timestanding Fermi-level pinning issue in fabricating III-V compoundsemiconductor MOSFET devices. As discussed in the context of FIG. 2,this issue is addressed in embodiments of the instant invention.

FIGS. 2( a)-(b) illustrate examples of InP MOSFETs. An InP MOSFET 200may be formed as follows. First, an appropriately doped orsemi-insulating semiconductor substrate with an InP-based channel layeris provided, i.e., an InP-based layer 110, over a substrate 100.Optionally, an additional epitaxial layer 200 may be deposited above theInP channel layer as described above for FIG. 1., e.g., by CVD, MOCVD orMBE. This additional epitaxial layer 200 may be InP and may includemultiple layers of a material other than InP, such as InSb, InAs, GaAs,InAlAs, and/or InGaAs that may be substantially lattice-mismatched toInP. In order to maintain strain in the epitaxial layer 200, itsthickness may be below the critical thickness.

An appropriate source and drain ion implantation is performed to createsource and drain regions 150, 160 that have a conduction type oppositeto that of the channel region, i.e., with n-type source and drain for ap-type channel (for an n-type inversion-type MOSFET), or with p-typesource and drain for an n-type channel (for a p-type inversion-typeMOSFET). For example, an appropriate p-type source and drainimplantation may be a Group II element such as Mg, Be, etc., with adosage ranging from, e.g., 4×10¹⁷/cm³ to 1×10¹⁹/cm³, with animplantation depth of, e.g., 1 nm to 1 μm. An appropriate n-type sourceand drain implantation may be a Group IV element, e.g., Si, Ge, etc.with a dosage ranging from, e.g., 4×10¹⁷/cm³ to 1×10¹⁹/cm³ with animplantation depth of, e.g., 1 nm to 1 μm. A target for both n-type andp-type source and drain implantations is to achieve dosage levels ashigh as are typically attained in Si devices. This may be a challenge inInP-based materials because n-type dopants in a III-V material mayexhibit p-type behavior above a certain concentration. For example,n-type dopant Si, when present in a concentration greater than about1×10¹⁹/cm³ in InP, exhibits p-type behavior. A high-quality gatedielectric 210 is formed, typically by first performing asurface-preparation step including a liquid-phase pretreatment or agas-phase pretreatment employing, e.g., (NH₄)₂S or (NH₄)OH, followed bysurface treatment with hydrogen or nitrogen plasma, either in-situ inthe same chamber, if feasible, or ex-situ. Preferably an ALD process isused to deposit a high-k dielectric layer. The gate dielectric layer maybe an Al₂O₃ layer, an HfO₂ layer, a nanolaminate layer of HfO₂ andHfAlO, a group-II oxide, or any other dielectric material such as arare-earth oxide or metallic oxide. The gate dielectric layer may have athickness selected from a range of 1 nm to 1000 nm, preferably 1 nm-10nm. The gate 220 is defined by conventional gate electrode depositionand lithographic patterning. Subsequently, source and drain ohmiccontacts are formed.

FIG. 2( b) illustrates the structure of a prototype inversion-type InPMOSFET 230, integrated with a high-k dielectric deposited by ALD. Thesubstrate 100 may be an InP semi-insulating substrate. The source anddrain regions 150, 160 are defined over an Si implanted n+ region 240.The gate dielectric 210 may be formed from a high-k dielectric, such asAl₂O₃, HfO₂, or HfAlO, and/or combinations thereof. The gate 220 isformed from a suitable conducting material, such as nickel/gold.

The InP MOSFET 230 operates as an inversion-type, enhancement modedevice. Because the source and drain regions are of a doping typeopposite to that of the channel, e.g., n-type source and drain regionsand a p-type channel, at a gate voltage V_(g)=0, the device is off, suchthat no current flows between the source and drain.

FIG. 2( c) summarizes the key steps for the fabrication of Al₂O₃/InPMOSFETs by use of ALD on an InP semi-insulating substrate, asillustrated in FIG. 2( b). Prior to the formation of the high-k gatedielectric 210, surface cleaning and preparation of the substrate 100includes HF- or HCl-based native oxide removal, NH₄OH-based native oxidecleaning and OH-group passivation, and (NH₄)₂S based sulfur passivation.These ex-situ wet-cleaning process steps may be extended to an in-situgap-phase dry-cleaning process that comprises HF or HCl gas-phasecleaning, NH₄OH gas-phase cleaning, and (NH₄)₂S gas-phase passivation.Additionally, acetone, methanol, and iso-proponal cleaning may beperformed.

After surface degreasing and (NH₄)₂S-based pretreatment, the substrateis transferred in a room ambient to an ALD reactor, such as an ASM F-120ALD reactor, for gate dielectric 210 formation. The gate dielectric 210may be, for example, an Al₂O₃ layer having a thickness of, e.g., 30 nm,deposited at a substrate temperature of 300° C., using alternatelypulsed chemical precursors of Al(CH₃)₃ (the Al precursor) and H₂O (theoxygen precursor) in a carrier N₂ gas flow.

Source and drain regions 150, 160 are selectively implanted through the30 nm thick Al₂O₃ layer, e.g., with a Si dose of 1×10¹⁴ cm⁻² at 140 keV.Implantation activation may be achieved by rapid thermal anneal (RTA),e.g., at 720° C. for 10 seconds. Regrown oxide may be etched away usingBHF. A thin layer of Al₂O₃ or HfO₂ or HfAlO (e.g., 8 nm) is deposited,and a post deposition anneal is performed. Those of skill in the art arefamiliar with suitable alternative materials and process parameters forsurface preparation and implantation and activation of source and drainregions.

The source and drain ohmic contacts may be defined by an electron beamevaporation of a combination of AuGe/Pt/Au and a lift-off process,followed by a RTA process at 500° C. for 30 seconds, also in an N₂ambient.

The gate electrode may be defined by electron beam evaporation ofconductive materials, such as Ni/Au, Ti/Au, AuGe/Ni/Au, and/orAuGe/Pt/Au and a lift-off process.

Referring now to FIGS. 2( d)-(j), data for an enhancement-mode n-channelInP MOSFET fabricated on a semi-insulating substrate with (ALD) Al₂O₃ asthe gate dielectric are shown. The results illustrated in FIGS. 2(d)-(j) are for MOSFETs with nominal gate lengths varying from 0.75 μm to40 μm and a gate width of 100 μm. Transfer-length-method (TLM)structures are used to determine contact resistance of 2.5ohm-millimeter (Ω·mm) and sheet resistance of 230 Ω/sq. at the implantedarea. Measurements are made with standard equipment such as an HP4284LCR meter for the capacitance measurement and a Keithley 4200 formeasurement of the MOSFET output characteristics.

FIG. 2( d) illustrates the drain current vs. drain bias as a function ofgate bias for a 1 μm InP MOSFET with 8 nm regrown Al₂O₃ as gatedielectric. In particular, the figure illustrates the dc I_(ds)-V_(ds)characteristic with a gate bias from 0 to 8 V. The measured MOSFET has adesigned gate length at mask level (LMask) of 0.75 μm and a gate width(L_(w)) of 100 μm. A maximum drain current of 70 mA/mm is obtained at agate bias of 8 V and a drain bias of 3 V. The gate leakage current isbelow 10 μA/mm under the same bias condition, more than 4 orders ofmagnitude smaller than the drain on-current. A maximum transconductanceg_(m) is approximately 10 millisiemens/mm (mS/mm) and an outputconductance of approximately 3 mS/mm (V_(g)=8 V). The relatively lowg_(m) may be improved by reducing the thickness of the dielectric,increasing the dielectric constant, and improving the quality of theinterface.

Since the fabrication process used in some embodiments is notself-aligned, accurate determination of the effective gate length andseries resistance is especially important for evaluating the intrinsicdevice performance and the potential for further optimization. FIG. 2(e) illustrates measured channel resistance vs. mask gate length fordifferent gate biases with Al₂O₃ thickness of 30 nm. Specifically, FIG.2( e) shows the effective gate length (L_(eff)) and series resistance(R_(SD)) extracted by plotting channel resistance R_(Ch) vs. L_(Mask).R_(SD) and ΔL, which is the difference between L_(Mask) and L_(eff), aredetermined to be 38.6 Ω·mm and 0.5 μm, respectively, by the equationbelow:

$\begin{matrix}\begin{matrix}{\frac{V_{ds}}{I_{d}} = R_{Ch}} \\{= \frac{L_{eff}}{W\;\mu_{eff}{C_{G}( {V_{GS} - V_{T}} )}}} \\{= \frac{L_{Mask} - {\Delta\; L}}{W\;\mu_{eff}{C_{G}( {V_{GS} - V_{T}} )}}}\end{matrix} & (1)\end{matrix}$The effective electron mobility μ_(eff) is weakly dependent on gate biasfrom 2-4 V and is taken as a constant in this calculation. R_(SD) andL_(eff) is determined as the intercept of the linear fitting of R_(Ch)at different gate biases and L_(mask) as shown in FIG. 2( e). The R_(SD)value of 386Ω and the ΔL value of 0.5 μm were determined using linefitting. The obtained R_(SD) is consistent with the results frommeasurement by a TLM technique since R_(SD) includes contact resistance,sheet resistance, accumulation resistance and spread resistance. ΔL iscaused by the inter-diffusion of source and drain implant activation andthe proximity effect of the photolithography process.

FIG. 2( f) illustrates extrinsic and intrinsic drain current andtrans-conductance vs. gate bias. In particular, to evaluate the outputcharacteristics more accurately, the intrinsic transfer characteristicsis calculated by subtracting the series resistance R_(SD) and usingeffective gate length L_(eff) instead of mask gate length L_(mask) andcompared with the extrinsic one, as shown in FIG. 2( f). The intrinsicdrain current and transconductance are only about 10% larger than theextrinsic ones due to the large gate length of 20 μm. FIG. 2( f) alsoshows that the subthreshold characteristic is scarcely changed. However,the threshold voltage determined by conventional method of linear regionextrapolation does show some difference as highlighted as extrinsicthreshold voltage V_(T) and intrinsic threshold voltage V_(T)* in FIG.2( b).

To better extract the threshold voltage, which is an important parameterin E-mode device characterization, several different methods are used todetermine V_(T)* on various gate lengths as presented in FIG. 2( g).FIG. 2( g) illustrates threshold voltage vs. the gate length using theintrinsic I-V characteristics of the device using five differentmethods—the ratio method (RM), the extrapolation in the linear regionmethod (ELR), the second derivative method (SD), the second derivativelogarithmic method (SDL), and the match-point method (MP). The figureshows that the linear method may not be appropriate for determinethreshold voltage for a non-self-aligned process. The second derivativemethod and ratio method give mostly the same value at the long gatelength device and both show V_(T)* roll-off behavior for the submicrongate length device. These two methods may be more reliable ways todetermine threshold voltage since the conducting mechanism changesduring the transition near threshold, when drain current increases fromexponentially to linear or quadratic dependence on the effective gatevoltage (V_(g)-V_(T)).

FIG. 2( h) illustrates drain current vs. drain bias as a function ofgate bias for a 2 μm InP MOSFET with 8 nm Al₂O₃ gate oxide. Inparticular, the sub-threshold slope (S.S>) and DIBL characteristics of280 mV/dec and 50 mV for the MOSFET are illustrated.

FIG. 2( i) illustrates effective mobility vs. normal electric field forthe InP MOSFET with 30 nm Al₂O₃ gate oxide and a 100 kHz split-CVmeasurement. Effective mobility is another important parameter forevaluating MOSFET performance. The “Split-CV” method is used to measurethe channel capacitance, which may be used to calculate the totalinversion charge in the channel by integrating the C-V curve. The insetof FIG. 2( i) is a 100 kHz C-V curve between gate and channel measuredon a 40 μm gate length device from which inversion capacitance is seenclearly. The extracted mobility has a peak value of 650 cm²/Vs around anormal electric field of 0.22 MV/cm as shown in FIG. 2( i). Bettermobility performance may be achievable by using an epitaxial InP layerto improve host material quality and optimize the device fabricationprocess.

Detailed C-V measurements of MOS capacitors were carried out to evaluatethe interface quality of Al₂O₃ formed by ALD on InP. FIG. 2( j)illustrates C-V measurements of an 8 nm Al₂O₃/n-InP MOSCAP fromquasi-static up to 1 MHz.

Specifically, the samples include 8 nm thick Al₂O₃ deposited on ann-type InP substrate at 300° C. by ALD. A 500° C. post depositionannealing improves C-V characteristics only moderately for InP as shownin FIG. 2( j), in contrast to the GaAs case. The frequency dispersion ataccumulation capacitance may be attributed to the relative highinterface trap density (D_(it)) at the conduction band edge, though theextrinsic parasitic effects could also contribute to the frequencydispersion partly. The mid-gap D_(it) is estimated to be approximately2-3×10¹²/cm²-eV determined by the high-frequency (HF)−low-frequency (LF)method. This value is consistent with the value determined from them-factor, a parameter related to sub-threshold characteristics. Morespecifically, the subthreshold slope is relatively large due to thelarge gate oxide thickness or the small oxide capacitance C_(ox) and theexisting interface trap capacitance C_(it). M-factor is defined as 60mV/dec.×(1+C_(it)/C_(ox)). From the measured sub-threshold slope, aninterface trap density D_(it) of ˜2-3×10¹²/cm²-eV is determined.

Moderate hysteresis of 100-300 mV exhibits in the C-V loops (not shown).The C-V characteristics in FIG. 2( j) show a clear transition fromaccumulation to depletion for HF C-V and the inversion features for LFC-V and quasi-static C-V indicating that the conventional Fermi-levelpinning phenomenon reported in the literature is overcome in this ALDAl₂O₃/InP sample. The unpinning of the Fermi level may be attributed tothe self-cleaning ALD Al₂O₃ process that removes the native oxide on theInP surface, similarly to the situation in ALD Al₂O₃ formed on GaAs. Theunpinning of the Fermi level by the ALD Al₂O₃ process described above issignificant, as it contributes to the realization of enhancement-modeMOSFETs on InP, as demonstrated in FIG. 2( d).

The ALD process on III-V compound semiconductors enables the formationof high-quality gate dielectrics and unpinning of the Fermi level oncompound semiconductors. A 0.75-μm gate-length E-mode n-channel MOSFETwith an Al₂O₃ gate oxide thickness of 30 nm may have a gate leakagecurrent less than 10 μA/mm at a gate bias of 8 V, a maximum draincurrent of 70 mA/mm, and a transconductance of 10 mS/mm. The peakeffective mobility is ˜650 cm²/Vs and the interface trap density ofAl₂O₃/InP is estimated to be ˜2-3×10¹²/cm² eV.

The prototype measurements reflected in FIGS. 2( d)-(j) reflect the useof Al₂O₃ as the gate dielectric. Alternatively other materials, such asHf-based high-k materials, can be used for the gate dielectric. Anexemplary process for formation of HfO₂ as a dielectric by using ALD isto use ALD precursors such as HfCl₄, TEMAHf or TDMAHf and oxygenprecursors such as H₂O or O₃. A typical HfO₂ ALD process is performedbetween 200-350° C. Surface cleaning and preparation is similar with useof materials such as HF, HCl, NH₄OH and (NH₄)₂S.

For InP devices, such as the examples of FIGS. 2( a)-(j), the substratemay be a semi-insulating InP substrate or alternatively may be anothermaterial that supports an InP channel device. For example, a Si or SOIsubstrate, can provide cost and manufacturing advantages because of thepossibility of using large-scale wafers and the well-developed Si-basedprocessing facilities.

InP-based devices may be integrated on Si substrates by the use ofaspect ratio trapping (ART) techniques. Two exemplary approaches forsuch integration using ART techniques are illustrated in FIGS. 3(a)-(d). A detailed description of ART techniques may be found in U.S.patent application Ser. No. 11/728,032, incorporated herein in itsentirety.

FIGS. 3( a)-3(b) show how ART techniques may be used to grow ahigh-quality InP layer above a buffer layer on a lattice-mismatchedunderlying material, such as a Si wafer. The crystalline substrate mayhave a lattice constant substantially different from a lattice consantof InP. As illustrated in FIG. 3( a), a non-crystalline material, e.g.,a dielectric layer 300, is formed over the substrate 100. The dielectriclayer 300 may include a dielectric material, such as silicon nitride orsilicon dioxide. The dielectric layer 300 may be formed by a methodknown to one of skill in the art, e.g., thermal oxidation orplasma-enhanced chemical vapor deposition. As illustrated, thedielectric layer 300 has a thickness t₁ corresponding to a desiredheight of crystalline material to be deposited in a window 310 formedthrough the dielectric layer 300. In some embodiments, the thickness t₁of the dielectric layer 300 is selected from a range of, e.g., 20-50000nm.

A mask (not shown), such as a photoresist mask, is formed over thesubstrate 100 and the dielectric layer 300. The mask is patterned toexpose at least a portion of the dielectric layer 300. The exposedportion of the dielectric layer 300 is removed by, e.g., reactive ionetching (RIE) to define the window 310 in the non-crystalline mask,i.e., in the dielectric layer 300. The window 310 extends to a surfaceof the substrate 100 and may be defined by at least one sidewall 320.The sidewall 320 is formed from the dielectric layer 300 and is,therefore, non-crystalline. The sidewall 320 may have a height h atleast equal to a predetermined distance H from the surface of thesubstrate 100, i.e., the sidewall extends above the substrate by theheight h. It has been observed experimentally that dislocations in amismatched cubic semiconductor grown on a Si (100) surface in thenear-vicinity (e.g., within approximately 500 nm or less) of a verticaldielectric sidewall 420 surface bend toward that surface atapproximately 30 degrees through 60 degrees. For example, thedislocations may bend toward that surface at approximately a 45 degreeangle to that surface. Based on this relationship, one may typicallyexpect the predetermined distance H necessary to trap defects to beapproximately equal to a width between ½ w and 2w, where w is the widthof the window.

The window 310 may be substantially rectangular in terms ofcross-sectional profile, a top view, or both, and have a width w that issmaller than a length l (not shown) of the window. For example, thewidth w of the window may be less than about 5000 nm, e.g., about20-1000 nm. In some embodiments, the width of the window is about 150nm. A length l of the window may exceed each of w and H. The ratio ofthe height h of the window to the width w of the window may be ≧1, andpreferably is between about 1 and about 50.

A crystalline buffer layer 330 including a material, such as Ge or InP,that has a lattice mismatch with the substrate 100 material, e.g., Si,is epitaxially grown within the opening 310. By configuring the aspectratio of the height of the mask relative to the width of the opening,defects arising from deposition of the buffer layer may exit at thesidewalls below the top surface of the mask layer. This provides ahigh-quality top surface of the buffer layer 330 upon which theInP-based top layer 110 may be grown epitaxially.

In particular, the buffer layer 330 may include a second crystallinesemiconductor material that may include or consist essentially of agroup IV element or compound, a III-V compound, and/or a II-VI compound.Examples of suitable group IV elements or compounds include germanium,silicon germanium, and silicon carbide. Examples of suitable III-Vcompounds include gallium arsenide, gallium nitride, indium arsenide,indium antimonide, indium aluminum antimonide, indium aluminum arsenide,indium phosphide, and indium gallium arsenide. Examples of suitableII-VI compounds include zinc selenide and zinc oxide. The buffer layer330 is lattice-matched to the InP-based top layer 110 formed thereover,i.e., the buffer layer 330 may have a second lattice constantsubstantially similar to the lattice constant of InP.

The second crystalline semiconductor material may be formed by selectiveepitaxial growth in any suitable epitaxial deposition system, including,but not limited to, MOCVD, atmospheric-pressure CVD (APCVD), low- (orreduced-) pressure CVD (LPCVD), ultra-high-vacuum CVD (UHCVD), MBE, orALD. In the CVD process, selective epitaxial growth typically includesintroducing a source gas into the chamber. The source gas may include atleast one precursor gas and a carrier gas, such as, for example,hydrogen. The reactor chamber may be heated by, for example, RF-heating.The growth temperature in the chamber may range from about 300° C. toabout 900° C., depending on the composition of the epitaxial region. Thegrowth system may also utilize low-energy plasma to enhance the layergrowth kinetics. CVD has a number of advantages, including thecapability for depositing films with low defect densities and rapidlyvarying alloy compositions, as well as high quality regrowth capability.CVD may also provide improved manufacturability due to relatively higherthroughput, relatively short downtimes, and scalability to very largereactors.

The epitaxial growth system may be a single-wafer reactor or amultiple-wafer batch reactor. Suitable CVD systems commonly used forvolume epitaxy in manufacturing applications include, for example, anAixtron 2600 multi-wafer system available from Aixtron, based in Aachen,Germany; an EPI CENTURA single-wafer multi-chamber systems availablefrom Applied Materials of Santa Clara, Calif.; or EPSILON single-waferepitaxial reactors available from ASM International based in Bilthoven,The Netherlands.

Dislocation defects in the second crystalline semiconductor materialreach and terminate at the sidewalls 320 of the window in the dielectricmaterial 300 at or below a vertical predetermined distance H from thesurface of the substrate, such that dislocations in the secondcrystalline semiconductor material decrease in density with increasingdistance from the bottom portion of the window. Accordingly, the upperportion of the buffer layer 330 is substantially exhausted ofdislocation defects. Various dislocation defects such as threadingdislocations, stacking faults, twin boundaries, or anti-phase boundariesmay thus be substantially eliminated from the upper portion of theepitaxial region. A density of such dislocation defects may be lessthan, for example, 10⁶/cm², preferably less than 10³/cm². The secondcrystalline semiconductor material may be either substantially relaxedor strained.

The InP-based top layer 110, i.e., the channel layer, is formed over thebuffer layer 330, the channel layer being substantially lattice-matchedwith the buffer layer. After using ART techniques to provide ahigh-quality InP-based top layer 110, the techniques as described abovein connection with FIGS. 1 and 2 may be used to create a device 340,such as a transistor, which incorporates the InP-based top layer,preferably as a channel. The source and drain regions of the transistormay also be formed in the channel layer, and a gate formed above thechannel layer.

Although using a Si substrate provides cost and manufacturabilityadvantages, ART techniques may be used with a variety of substrate andmask materials. As discussed in U.S. Ser. No. 11/728,032, the openingmay be configured in a variety of ways based on materials andapplications, such as in a trench configuration.

Referring now to FIGS. 3( c)-(d), an alternative ART approach forproviding an InP-based channel layer is described. The mask 300 definesa plurality of openings 310 above substrate 100. An InP-based material,such as InP, is epitaxially grown within the openings, which areconfigured with an aspect ratio so that the majority of defects arisingfrom the lattice mismatch between the underlying substrate, e.g., Si andInP, exit the InP-based layer at the mask sidewalls within the openings.The InP-based material is epitaxially grown to extend above the mask 300to define a high quality coalesced InP-based top layer 110, upon whichInP-based devices 340, such as the MOSFETs described above withreference to FIGS. 1 and 2, may be fabricated.

Creation and activation of the source and drain may be a challengingstep in forming transistors such as an inversion-type InP MOSFET. Analternative to the implanted source/drain formation approach discussedabove uses a replacement source/drain structure. Referring to FIG. 4(a), a semiconductor substrate 100 with an InP-based top channel layer110 is provided, with predefined channel doping type and doping level.The channel may be, for example, p-type doped with a doping level of1×10¹⁸/cm³ to 1×10¹⁹/cm³. For a III-V material such as InP, a suitablen-type dopant is a Group IV element, such as Si. A suitable p-type is aGroup II element, such as Mg. A relatively high doping level ispreferred for enabling the low contact resistances. It is a challenge,however, to dope InP and other III-V materials to a level greater than1×10²⁰/cm³.

A gate dielectric 210 is deposited on the InP channel material accordingto the methods described above, for example using ALD to deposit adielectric material such as Al₂O₃. The dielectric material may includehafnium, e.g, HfO₂. Thereafter, a conductive layer, e.g., a gateelectrode layer is formed over the gate dielectric, such that thedielectric layer is disposed between a semiconductor layer, i.e., theInP-based top channel layer 110, and the conductive layer. A gate 220 isdefined in the conductive layer by patterning methods known to one ofskill in the art.

Using techniques known to those familiar with the art, a source regionand a drain region are patterned in a lithographic mask next to the gatestructure, and the portions of the InP-based top layer 110 in the sourceand drain regions are removed by, e.g., dry etching, to form source anddrain recesses 400, 400′, as shown in FIG. 4( b). The source and drainrecesses 400, 400′ are filled with an appropriate source/drain material410 that has a doping type opposite to that of the channel region, asshown in FIG. 4( c). In an embodiment, InP may be grown epitaxiallywithin the source and drain recesses 400, 400′ by CVD with dopingperformed in situ. With a p-type channel dopant, an n-type dopant may beused for the source and drain to create an n-MOSFET. The doping typesare reversed for a p-MOSFET. Those of skill in the art understand how touse appropriate dopants and materials to achieve the desired devicecharacteristics.

This approach, including the regrowth of the source and drain in therecesses, may allow the achievement of higher doping concentrations inthe source and drain regions, and may enable lower contact resistances.The MOSFET structure illustrated in FIG. 4 c may be an inversion-type,enhancement-mode device. As discussed previously, at Vg=0, a pn-junctionblockades the drain current in the channel so that the device is off. AtVg>0, the InP-based channel under the gate is inverted to become n-type,so the device is on with a large drain current.

FIGS. 5( a)-(g) illustrate a method of creating a MOSFET with source anddrain regions that are self-aligned to the gate, i.e., the edges of thesource and drain regions next to the gate are defined by the same maskthat defines the edges of the gate next to the source and drain regions.Referring to FIG. 5( a), a mask 500 defines the locations of the gateand channel locations over a substrate 100. The mask 500 functions as adummy gate and also as a barrier to ion implantation in the channelregion during creation of the source and drain regions 510, 520 by ionimplantation, as shown in FIG. 5( b). The mask may be made from anappropriate material, such as photoresist. After ion implantation of thesource and drain regions, an insulator 530 such as SiN is deposited overthe mask 500 and source and drain regions 510, 520, and is then etchedback to expose a top surface of the mask 500 (i.e., of the dummy gate)as shown in FIGS. 5( c)-5(d). Referring to FIG. 5( e), the dummy gate isthen removed by a selective etch, e.g., by a wet etch such as KOH orTMAH, or a dry etch in a reactive ion etching system, to define anopening 540. A gate dielectric 550 and gate material layers 560 aredeposited into the opening created by the removal of the dummy gate, andetched back as shown in FIGS. 5( f)-(g). Advantageously, thisself-aligned gate procedure enables the use of InP as a channel materialand ALD for depositing a high-k gate dielectric while maintaining small(sub-micron) feature sizes and spacing. Forming a dummy gate enables theformation of a self-aligned gate, without exposure of the device gatedielectric and gate electrode to a high temperature dopant activationprocess that may damage a high-k dielectric. The dummy gate approachincludes forming a gate dielectric after a source/drain implantation andactivation, rather than before these steps, as in a conventional processflow.

InP is a binary compound semiconductor resulting from a relatively weakcovalent bond between indium and phosphorous. While InP MOSFETs featurecharacteristics such as enhanced drain currents and improved performancewith respect to Fermi-level pinning and higher electron saturationvelocity, the relatively strong electropositivity of indium can be adisadvantage, resulting, for example, in the relatively high reactivityof InP with oxygen. Thus, for example, as shown in FIGS. 6( a)-6(b), tomitigate interfacial instability difficulties between InP and dielectricmaterials as well as to prevent incongruent sublimation of the III-Vmaterial InP, it is possible to use a diffusion/barrier layer 600including a relatively inert materials such as transition metal nitridesor group III-nitrides (e.g., BN, AlN, MoN, TaN, TiN, or AlN, etc., whichare much less reactive with InP than oxygen, especially at highertemperatures) adjacent the top surface of the InP-based top layer 110 ofsubstrate 100. As shown in FIG. 6( b), the use of a relatively inertdiffusion/barrier layer 600 permits the sequential deposition of arelatively non-inert dielectric 610 such as SiO₂ or Al₂O₃ above therelatively inert layer. By use of ALD, the inert barrier layer 600 maybe kept relatively thin, e.g., as thin as one monolayer (which typicallywill be about 15 angstroms (Å)), or preferably about 5 to 10 nanometers(nm), although the barrier layer can be substantially thicker, e.g., asthick as 1 μm or more. In alternative embodiments, the barrier layer maybe AlN or AlON or a nanolaminate—e.g., alternating layers of AlN andAl₂O₃, deposited as monolayers or in layers of about 5 nm—which canenhance the ability of the barrier layer to trap materials such as In orP before they can diffuse through the barrier layer.

FIGS. 7( a)-(d) illustrate a method of forming source and drain regionsthat also provides for an improved dielectric layer, e.g., an improvedgate dielectric layer. In FIG. 7( a) a dielectric 700 inert to InP, suchas a transition metal-nitride, is provided above an InP-based top layer110 using ALD as described above for FIG. 6. In FIG. 7( b) the gate 710is defined and the source and drain regions 720, 730 of a transistor areimplanted using techniques similar to those discussed above for FIGS. 1and 2, and the source and drain regions are amorphized. In anembodiment, amorphization is accomplished by using a heavy dopantspecies such as SiF_(x), with sufficient dopant dosage and energy toconcomitantly accomplish both amorphization and high dopantconcentration of the source/drain regions. In another embodiment,amorphization is accomplished by implanting an inert element such asargon or another suitable noble gas prior to the implantation of thedopant species. In still another embodiment, amorphization isaccomplished by using a heavy dopant species such as SiF_(x), withsufficient dopant dosage and energy to concomitantly accomplishamorphization of and high dopant concentration in the source/drainregions. For example, complete amorphization of the InP lattice may beaccomplished through the use of high dosage implants using compoundssuch as SiF_(x) between 10¹⁴ cm⁻² and 10¹⁶ cm⁻² through dielectriclayer(s). Easy displacement of the In sublattice along <0001> directionscan enable amorphization through heavy dosage implantation.

In FIG. 7( c) the source/drain regions 720, 730 are annealed at atemperature sufficiently high to ensure recrystallization, such as byRTA between 400° C. and 1000° C. Use of high implantation dosages andimproved activation/ionization of the dopant species throughrecrystallization of the amorphized InP lattice lowers the contactresistance. Then in FIG. 7( d) an oxidizing species, e.g., PECVD SiO₂,is introduced at a temperature of, e.g., 300° C., to enable formation ofan oxynitride from the dielectric 700, e.g., from a dielectrictransition metal nitride. Converting the dielectric transition metalnitride into an oxynitride provides an improved dielectric from the samelayer that provided an effective diffusion barrier, eliminating the needto deposit new dielectric material. Effective performance of thetransistor embodiment of FIG. 7 is enhanced by the use of ohmiccontacts.

In some embodiments, processes and materials analogous to the methodsdescribed above may be used to form a high electron mobility transistor(HEMT) including an InP-based material. A HEMT is a field effecttransistor incorporating a junction between two materials with differentband gaps (i.e., a heterojunction) as the channel instead of a dopedregion. An InP-based material may be incorporated in a HEMT as a portionof the channel layer.

The embodiments disclosed above and their variations address a varietyof challenges in providing enhanced MOSFET performance in a commerciallyviable way through various combinations of materials and techniques suchas InP-based, ALD, and Al₂O₃ or Hf-based dielectrics in conjunction withvarious other features and steps. Those skilled in the art willunderstand how to substitute other materials and process steps to applythese inventive combinations to a variety of applications.

1. A method of fabricating a transistor, the method comprising:providing a crystalline substrate having a first lattice constantsubstantially different from a lattice constant of InP; providing anon-crystalline mask above the substrate, the mask including an openinghaving sidewalls that extend above the substrate by a height; providingwithin the opening a crystalline buffer layer having a second latticeconstant substantially lattice-matched to InP, the buffer layer having aheight less than the mask opening sidewall height, the buffer layerheight being sufficient to permit defects arising from the latticemismatch to exit the buffer layer at the mask opening sidewalls;providing a channel layer above the buffer layer, a portion of thechannel layer being lower than a top surface of the mask, the channellayer comprising a crystalline material, the crystalline materialcomprising an InP-based material; defining a source region, a drainregion, and a channel region of a transistor in the channel layer; andproviding a gate above the channel layer.
 2. The method of claim 1wherein the substrate is a silicon substrate.
 3. The method of claim 1wherein the mask comprises SiO₂.
 4. The method of claim 1 wherein thebuffer layer comprises a III-V material.
 5. The method of claim 1wherein the buffer layer comprises InP.
 6. The method of claim 1,wherein the source region and the drain region are defined in theInP-based material, the channel region being defined between the sourceregion and the drain region, the gate being provided above the channelregion, and further comprising depositing a gate dielectric over thechannel region by atomic layer deposition (ALD), wherein at least aportion of the gate dielectric is disposed between the gate and thechannel region and the transistor operates as an inversion-type,enhancement-mode device.
 7. The method of claim 6 wherein the gatedielectric comprises at least one of a group II material or a transitionmetal.
 8. The method of claim 7 wherein the gate dielectric comprisesaluminum.
 9. The method of claim 7 wherein the gate dielectric compriseshafnium.
 10. The method of claim 1 wherein the source and drain regionsare further disposed above the channel layer.
 11. The method of claim 1wherein the channel layer comprises a strained region.
 12. The method ofclaim 1 wherein the channel layer comprises at least one of InSb,InGaAs, or InAs.
 13. The method of claim 1 wherein the channel layercomprises a plurality of substantially lattice-matched layers.
 14. Themethod of claim 1 wherein the transistor is a MOSFET.
 15. The method ofclaim 1 wherein the transistor is a HEMT.
 16. The method of claim 1,wherein the providing the gate comprises: depositing by ALD, above atleast a portion of the channel layer, a dielectric layer comprisinghafnium; and providing a conductive layer over at least a portion of thedielectric layer, wherein the dielectric layer is disposed between thechannel layer and the conductive layer.